As a mechanism to transfer user traffic between edge devices within a carrier network, a label switching method using an MPLS (Multi-Protocol Label Switching) is generalized.
In the MPLS, each repeating device performs forwarding using an identifier which is attached to a packet and called a “label”. A path in which a packet to which this label is attached passes is called an LSP (Label Switched Path).
When a packet arrives at an LER (label edge router) at an entrance of the LSP, a label is added to path information in the packet, and transferred to an LSR (label switching router). The LSR reads the label of the input packet, judges to which output port to transfer and transmits the packet. An LER at an exit of the network removes the label from the packet arrived and transfers it to outside of the network.
As a path diagnostic technique in an LSP installed between edge devices in a carrier network, for example, a technology described in the following non-patent literature 1 exists. As indicated in paragraph 4.2.1 and paragraph 4.2.2 of non-patent literature 1, the path diagnostic technique related to the present invention transfers an OAM (Operation Administration and Maintenance) packet to an LSP which performs path diagnosis, and by confirming its reply, examines disagreement of a label transfer destination path built based on a label switching method.
That is, OAM packets [tracing (trace) packet of a request] of which an upper limit of the number of transfer hops of the packet are different are transmitted successively, and in case there is no reply, it is judged that there is a defect in the communication.
For example, as shown in FIG. 21, a device 2a transmits OAM packets (for example, request message of LSP-trace) of which an upper limit of the number of transfer hops (TTL: Time to live) of the packet are different.
In case a repeating device which receives the OAM packet mentioned above is structured as a switch of a chassis type, at input line cards 21a, 21b and 21c of the repeating devices mentioned above (2a, 2b and 2c), a TTL value of the OAM packet mentioned above is made “TTL=TTL−1”. In case result of this arithmetic operation is “0”, the OAM packet mentioned above is terminated at the input line card.
Further, in case target (Target) ID (IDentifier) information in its TLV (Type/Length/Value) coincides with its own card ID information, a normal reply message is generated and returned to the devices (repeating devices 2a, 2b and 2c).
Referring to a format of an OAM frame shown in FIG. 22, the OAM frame includes: an LSP header [including LSP label, EXP (Experimental field), S (the stacking bit indicates) and TTL information], an identification header for identifying whether it is a control message such as OAM, TLV which becomes a payload of the frame and so on.